on the shocking killing of the high school math teacher. A 14-year-old student you see there, under arrest right now. And abc's gio benitez is live in danvers, massachusetts, with the latest. Good morning, gio. Reporter: Good morning, robin. That beloved teacher, just 24 years old. Her accused murderer, a j.C. Soccer player. He had just transferred her from tennessee. Investigators say philip chism killed colleen ritzer, about 3:30 p.M. Tuesday. Reportedly, with a box cutter, in the school's faculty bathroom. And that surveillance cameras show him using what appears to be a recycling bin to move her body to the woods. She was just a young, caring girl, that had the whole world ahead of her. And to be taken just so tragically is awful. Reporter: By 6:00 p.M., Chism was reported missing. When he didn't show up to soccer practice, these students telling us, they searched for him all night. At first, we thought he was, you know, abducted or just gone missing. Reporter: Sources say he had gone to a nearby movie theater to watch a woody allen movie. By 11:00 p.M., Colleen ritzer was also reported missing. That's when police went to investigate and found her body. An hour and a half later, chism was spotted walking on the side of a busy road. And after questioning by police, arrested him. He has been charged as an adult, with ritzer's murder. His mother in tears. Chism pleading not guilty. Sadly, miss ritzer's final tweet, was about monday's school shooting in nevada. Another student allegedly killing a math teacher there. Ritzer called it simply devastating. She could find joy in anything. And she brought so much joy into my life. Reporter: At game one of the world series, a moment of silence in her honor. And this morning, we still don't know why this 14-year-old would have allegedly killed his own teacher, george. Gio, thanks. Let's get more from abc news brad garrett, special agent with the fbi. This is different from the shooting we saw earlier in the week. This time, not a random spree. The teacher, the sole target. This is a focused attack. I would assume this kid's motive might be sexual. It might be revenge. But it was focused just the teacher. And my sense is, the murder part of it, maybe secondary. That whatever he did, that this was a sexual assault, for example, that like so many people, they realize what they've done. And how much trouble they're in and atthey try to get rid of the evidence. You think the box cutter is significant. People tend to kill with items readily available, that the murder is secondary. He realized, once he got into this, he was in so much trouble, I have to get rid of this woman. That's when it happened. One thing investigators will be looking at, no doubt, possible mental health issues? I think so. People don't suddenly do. This it can be built up over months. You have to be big-time narcissistic and not be able to think about or have any empathy about other people to commit these types of acts. This guy had gone inside himself. And I think that's the big problem. Brad garrett, thanks very much.

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